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Small Block Chevy Cylinder Heads

As you may already be aware, there are is a dizzying number of cylinder heads available for the Chevrolet smallblock. For over 40 years Chevrolet has developed heads for all types of uses, including stock, high-performance, smog emissions and over-the-counter all-out race heads. Each was designed in its day to provide the type of service intended and reflected the technology and thinking of the times. Aftermarket head makers also have contributed to the design changes over the years. The sheer number of choices can be mind-numbing. Just keep in mind what your intentions are. That will help narrow the selection for you. Cast-iron heads are still cheaper than aluminum heads, but some aluminum heads are being offered at reasonable prices. If you are going to use unleaded pump gasoline for a street-driven machine, then the heads you build or purchase are going to need hard valve seats in order to survive. Most factory heads made before 1975 had soft seats that will have to be upgraded to hardened seats for today’s unleaded pump gasoline. Remember that different heads have different amounts of chamber volume. These volumes have a direct effect on the static compression ratio of the engine. What the compression ratio will be, in part, depends on which head casting you use.

The rocker arm valley on the 3947041 head has pressed in rocker studs. Head casting number and casting date of “I 11 8” can be seen, indicating this head was cast on September 11, 1968, and used in the 1969 model year. This particular closed chamber head was fitted with 1.94"/1.5" valves.

The head casting number on this “336X” head is located in the rocker arm valley.

Chevy has used various raised casting symbols on the ends of smallblock cylinder heads, which on some heads are duplicated in the rocker arm valley. These symbols were used to indicate the type of features found on those particular heads. The same symbol can be found on different heads with different casting numbers. These symbols are helpful, but should not be solely relied upon to determine what type of heads you have or what features they have. For instance, the camel hump symbol (also called double hump) can be found on 461, 461X, 462 and 291 casting heads. The 492, 292 and 186 heads have a slightly different looking camel hump. These heads may have come with 1.94"/1.5" valves or they might have been made with 2.02"/1.6" valves. The chamber shapes found on the 461 and 461X heads are not the same as those found on the 291, 462 or 186 heads. The 461X castings are popular for racers because they actually had slightly larger port volumes as cast than the 461 and other production heads. Some of these heads had accessory mounting holes, but heads made before ’69 do not. Just to make things interesting, another casting, the 041, was basically the same as the 186 heads, but did not have the camel hump casting symbol. The 041 heads have a completely different right angle symbol on their ends. The point is, you need to look at all of the identifiers on the head in order to know what you have. Some later head castings have the last three digits of the casting number stamped into a ledge on the ends of the head that are visible when the heads are on a motor. You may need a mirror to read them, because the ledge faces down when the head is on the block. End casting symbols give you some clues, but they don’t tell you the whole story.

This “336X” head also has the shift clock and the “D” and “N” for Day or Night shift and the rare “M” in the date code representing the month of December. On some casting dates Chevy felt that the use of the letter “I” for the month of September might be confused with a numeral “1.” So sometimes they skipped the “I” in the monthly alphabetical sequence and “J” was used for the month of September and then “M” was used for the month of December.

This “520” rocker valley shows the casting number and the Canadian manufacturing location.

The double hump or camel hump casting symbol on the end of this head indicates a performance cast-iron head used in the sixties. The lack of accessory bolt holes in this head narrows the possibilities as to which casting this is. Looking at the head casting number revealed that it was a “461” casting with 1.94"/1.5" valves that was found on a 327/300hp motor.

Shown is a “462” double camel hump casting symbol. These early heads did not have accessory bracket bolt holes.

Head casting identifier on “041” head is a symbol of a right angle triangle. However, this 1969 to ’70 high-performance head has the same characteristics as the 186 high-performance head, which has the double hump symbol. Both heads were available from the factory with 1.94"/1.5" or 2.02"/1.6" valve diameters depending on the application. They have soft seats.

The 76cc open combustion chamber found on the “993” casting on the left is noticeably different than the 64cc closed chamber 186 casting on the right. Both of these Gen. I heads came with 1.94"/1.5" valves.

Here is a “292” angle plug head showing the single camel hump casting identifier symbol and the accessory holes.

Smallblock heads have been made since 1955 and some of these heads are still in circulation. Many things could have been done to these used heads over the decades. They might have had hard seats from the factory or had hard seat inserts installed at a machine shop or may still have the soft seats they were originally made with. They may have been ported (correctly or incorrectly). They may have small diameter valves and pressed-in rocker arm studs or bigger valves and screw in studs may have been installed. The heads may be cracked (not uncommon even on newer heads). Someone may have added accessory bracket bolt holes to heads that didn’t originally have them (this is usually crudely and badly done). They may be in such bad shape that they are not rebuildable. Be careful. Some of these old heads are fit for use as boat anchors or hold downs for tent stakes, but not for use on any engine.

Various oil stem seals have been used on smallblock heads. The most common seal used on Gen. I heads is the small O-ring style shown on the right. The umbrella seal in the middle rear is found in many rebuild kits. The positive seals shown on the left slip over the valve guides and can also be used with double and triple valve springs since they will fit inside the innermost spring coils.

From 1955 to the ’59 model year, smallblock cylinder heads had a different staggered valve cover flange bolt pattern. The top two bolt holes were closer together than the lower bolt holes. Valve covers were made with bolt holes to match. In 1959, the valve cover holes in the heads were changed so that the holes were the same distance from one another across the top when compared to the lower holes. The 1959 and later valve cover holes were changed to match.

Note that the late-model heads are using a factory installed PC-type valve stem seal, instead of the old O-ring that was used for decades on the smallblock.

If you use a 400 block with steam holes in the deck, you need to use a 400 head gasket and cylinder heads with matching steam holes (arrows). If your heads don’t have the holes, you can use a 400 head gasket as a template and drill the six additional holes in each head yourself.

The single rear exhaust manifold bolt hole is seen in this six-hole “336X” head along with the boss for the water temperature sender.

When using the Gen. I long water pump, and its associated pulleys and accessory brackets, you need to use cylinder heads that have the necessary bolt holes in their ends (arrows). Heads cast from 1955 through ’68 don’t have them, and with the early heads, you need to use the short water pump, pulleys and accessory brackets.

In 1987, the way in which valve covers were attached to the heads was changed again. The covers bolt to bosses in the center of the rocker arm valley and the gasket was changed to accommodate machined valve cover rails on the heads. This was done to help eliminate once and for all the notorious valve cover oil leaks of previous designs. You can get an adapter (PN-24502540) that will allow the use of the 1959 to ’86 flange-mounted valve covers on the center-bolt ’87 and up heads.

During the 1968 model year, head castings were changed so that a boss could be mounted in the side of the head to mount the water temperature sending unit. Prior to that the temperature sending unit was mounted in the intake manifold and the heads did not have a boss for the temp sender. Some 1968 heads have the boss in the head, but the boss is not drilled or threaded.

In 1969, Chevy changed the way that engine accessory brackets were mounted. Prior to 1969, there are no holes in the ends of cylinder heads on which to bolt accessory brackets. From 1969 onward, the heads have threaded bosses and holes to mount the new style brackets. If you need these holes to use your brackets, make sure the heads you plan to use have them. If they don’t have accessory holes, use the earlier style, short water pump and pulleys with the related early style brackets for use with short style water pumps.

From 1970 through 1980, 400 smallblocks were installed in various vehicles. These engine blocks were different in that the cylinder barrels were siamesed in order to enlarge the cylinder bores to 4.125".

Take a close look at the spring package on these two Gen. I heads. The stock production head on the left has a slightly shorter exhaust spring that also uses a valve rotator (arrow). The head on the right uses intake and exhaust springs of the same length, but no valve rotator on the exhaust spring. The rotators are intended to turn the exhaust valves at low rpm and help keep the valve edges and seats clean. Don’t mix this up. If you are building a stock set of heads with the rotators, use the shorter exhaust spring. If you intend to use a larger than stock camshaft and increase the spring pressures, don’t use the rotators. Instead, install springs of equal length.

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